I currently have the same issue. The work around we have in place is to set the entire NSM Device to an inactivity setting of Days: 0 Hours: 0 Minutes: 0.
Obviously this isn't ideal but we utilize NSM's reporting to alert on errors in the system and there are alarms in place to alert if the devices connectivity or status changes.
To be honest, inactivity never worked for me in ESM. If it's working for you, please let me know what version of ESM you are using.